Electronic devices typically require a connected (wired) power source to operate, for example, battery power or a wired connection to a direct current (“DC”) or alternating current (“AC”) power source. Similarly, rechargeable battery-powered electronic devices are charged using a wired power-supply that connects the electronic device to a DC or AC power source. The limitation of these devices is the need to directly connect the device to a power source using wires.
Wireless power transfer (WPT) involves the use of time-varying magnetic fields to wirelessly transfer power from a source to a device. Faraday's law of magnetic induction provides that if a time-varying current is applied to one coil (e.g., a transmitter coil) a voltage will be induced in a nearby second coil (e.g., a receiver coil). The voltage induced in the receiver coil can then be rectified and filtered to generate a stable DC voltage for powering an electronic device or charging a battery. The receiver coil and associated circuitry for generating a DC voltage can be connected to or included within the electronic device itself such as a smartphone.
The Wireless Power Consortium (WPC) was established in 2008 to develop the Qi inductive power standard for charging and powering electronic devices. Powermat is another well-known standard for WPT developed by the Power Matters Alliance (PMA). The Qi and Powermat near-field standards operate in the frequency band of 100-400 kHz. The problem with near-field WPT technology is that typically only 5 Watts of power can be transferred over the short distance of 2 to 5 millimeters between a power source and an electronic device, though there are ongoing efforts to increase the power. For example, some concurrently developing standards achieve this by operating at much higher frequencies, such as 6.78 MHz or 13.56 MHz. Though they are called magnetic resonance methods instead of magnetic induction, they are based on the same underlying physics of magnetic induction. There also have been some market consolidation efforts to unite into larger organizations, such as the AirFuel Alliance consisting of PMA and the Rezence standard from the Alliance For Wireless Power (A4WP), but the technical aspects have remained largely unchanged.
Some techniques for WPT use two or more transmitter coils in an attempt to overcome the issue of low power transfer over short distances. Typically, two identical transmitter coils (e.g., both wound in the clockwise direction or both wound in the counter-clockwise direction and having the same number of turns and area) are coupled in series or parallel on a single magnetic layer to transfer power to a receiver coil. Alternatively, the coils can be placed in close proximity to one another without the use of a magnetic layer. This configuration results in the applied time-varying current flowing through both coils in the same direction at any point in time, generating an almost perpendicular combined magnetic field with flux lines that flow from both coils in the same direction (i.e., the magnetic field generated by either coil has the same polarity as the other coil). Magnetic flux lines tend to repel if they are in the same direction, which causes the flux lines to radiate through the air for great distances. When magnetic flux lines repel, the magnetic reluctance is high, resulting in a weak magnetic field that reduces the amount of magnetic coupling between the transmitter coils and a receiver coil placed in close proximity (i.e., 2-5 millimeters) to the transmitter coils. So although the coil area is larger than in a single-coil transmitter, the resulting magnetic flux available to transfer power is reduced. If the transmitter coils are placed on separate magnetic layers, an air gap exists between the magnetic layers resulting in an even weaker generated magnetic field as the air gap further increases the reluctance between the transmitter coils.
When multiple transmitter coils are used there is an additional problem with meeting Electromagnetic Interference (EMI) requirements, including, for example, spurious emissions requirements set forth by the Federal Communications Commission (FCC) and the European Telecommunications Standard Institute (ETSI). When magnetic flux lines radiate away from a transmitter coil into the environment, there is a higher likelihood that the emissions will exceed EMI compliance standards. Additionally, flux lines tend to radiate into the atmosphere if a transmitter coil is driven without a receiver coil to receive the generated flux. To avoid this issue, such multiple coil arrangements require polling the transmitter coils to determine which transmitter coils are providing power to a receiver coil (e.g., smartphone with a receiver coil placed over the transmitter coil) and disabling all other unused transmitter coils. If only a single transmitter coil is used to meet EMI requirements, this implementation will suffer from the same low power and short distance of power transfer issues discussed above.
Transmitters that comply with existing WPT standards also achieve substantially lower power transfer to an electronic device if the receiver coil in the electronic device is not properly centered over the transmitter coil. One technique used to address this issue is increasing the frequency of the time-varying current applied to the transmitter coil (e.g., frequencies above 400 kHz). Although higher frequencies of operation may increase the amount of power transferred over the distance between a transmitter coil and a receiver coil, higher frequencies of operation may interfere with the operation of other devices and may fail EMI compliance requirements by emitting energy in neighboring frequency bands that exceed spurious emission requirements.
Due to the short range of existing WPT technology, the transmitter coil must be centered with the receiver coil connected to a device and the coils cannot be more than 2-5 millimeters apart. This makes it difficult to implement wireless power transfer for devices that are not perfectly flat or do not have a large enough area for embedding a receiver coil (e.g., Android® wearable devices, Apple® watch, Fitbit® fitness tracker, etc.). The limitations of WPT also affect smartphones if the charging surface with the transmitter coil is not large enough to allow the smartphone device to sit flat on the surface (e.g., in vehicles, which typically do not have a large enough flat surface to accommodate a smartphone device). Thus, the current state of WPT technology is not suitable for many consumer devices.